Various environmental laws regulate the discharge or disposal of waste materials into natural streams, municipal sewers, land surfaces, or underground reservoirs since these materials usually contain hazardous or undesirable concentrations of contaminants. Generally, these environmental regulations prohibit the concentration levels of the contaminants from exceeding specific limits which have been determined to ensure a relatively safe environment. The disposal of materials that exceed these limits is a major problem of growing complexity particularly in view of ever increasing amounts of such materials for disposal and in view of stricter state and federal environmental regulations. There are many industrial and commercial sites in need of remediation either because of ongoing operations that generate hazardous waste materials that must be disposed of or because of accumulated waste materials that have not been processed to reduce the contaminants to meet the proscribed limits. The clean-up of contaminated industrial sites in particular has heretofore presented problems primarily because of the expense and time involved and the tremendous energy requirements that accompany the methods that have been available.
Conventional methods for the cleanup of contaminated sites include: (1) dewatering of sludges, treating the liquid phase, and land disposing the solid phase (filtercake); (2) vaulting in place using slurry walls and stabilization agents; (3) incineration; and (4) thermal desorption. Methods (1) and (2) can be eliminated if the material is covered by EPA Land Disposal Restrictions (LDR) which require that these LDR waste materials meet the applicable treatment standard before subsequent placement in or on the land. If LDR applies, incineration is usually the selected method of disposal. However, incineration is very unpopular with the public often creating additional environmental concerns. For example, incineration in the presence of water and oxygen may result in the formation of dioxins/furans which are known to be extremely toxic and in all likelihood are more hazardous than the initial contaminant that was incinerated. As a result, incineration permits for off-site processing are often either unavailable or very difficult to obtain. Moreover, even when such permits are obtained, on-site incineration may add up to 50% to 300% of additional expenses to the cleanup costs, due to large volumes of combustion gases that will require air handling and treatment equipment such as water quenchers, dust collectors, and water scrubbers. Moreover, the wastes (e.g.,ash) from the incineration process may require disposal in a secured land fill. Since incineration generally results in little volume reduction, disposal costs for the waste ash can be significant. Off-site incineration can add additional costs associated with packaging and transporting the waste materials to the incineration site. Off-site incineration can add up to 30% to 50% of additional costs to the on-site incineration costs. Additionally, off-site incineration can add months to the completion of the cleanup due to low incineration capacity nationwide. Both on-site and off-site incineration can result in residual wastes that are covered by RCRA, resulting in additional encapsulation costs for disposal of the residual wastes generated by the incineration.
The provision of a method for cleaning up sludges or contaminated soils or other contaminated solid waste materials that does not involve on-site or off-site incineration is an increasingly critical need in the art.
Thermal desorption is a physical separation process employed for the removal of organics from many types of solid materials such as soil, sludge, and filtercake, which is typically carried out using a direct fired rotary dryer followed by a baghouse, thermal oxidizer (afterburner or incinerator for gases), water quench to cool the gases, packed scrubber, and stack for emission of gases. Additionally, various systems have been proposed that use direct or indirect heat exchange to achieve bed temperatures of 200.degree. to 1000.degree. F. with oil or combustion gas as the transfer medium. Residence times typically depend on contaminant vapor pressures, soil conditions, operable variables, and applicable remediation criteria. Residual treatment requirements typically include those for oversized reject, offgases and water. Particulate control systems are also required. Offgas treatment typically includes both thermal oxidation (at temperatures above 1,600.degree. F.) to burn the organics and scrubbing units to remove chemicals such as HCl. Such off-gas treatment is generally significant in terms of cost and meeting regulatory requirements. Although thermal desorption is a proven method for the removal of organics, regulatory agencies often require significant treatability testing and other measures (e.g., test burns) to ensure that there are no emissions problems and that the organics are not transformed into more toxic products as a result of the thermal desorption or offgas treatment processes.
None of the known thermal desorption treatment methods use superheated steam in a portable, optionally continuous, optionally countercurrent or concurrent flow, multi-stage process and system with closed loop superheated steam recycle as described and claimed hereinbelow.
A thermal desorption process that has been proposed is that described in the publication Thermal Desorption by Steam Stripping/Solid Waste Desorption, Texarome, Inc., EPA SITE Technology Profile, pp.152-153, November, 1991; EPA VISITT, pp.1-14, Aug. 19, 1991; and related excerpt, pp.1-3. The Texarome process is described as a portable, continuous, countercurrent flow, multi-stage process with interstage dispersion and separation of the phases and one in which soils conveying and collection systems are isolated by piping systems of a proprietary design and process equipment which is then surrounded by an enclosure and jacket. A portion of the stack gases from the steam generated is used in the surrounding jacket and is moved through the system by an induced draft fan. Superheated steam is used to convey solids through the processing system and process equipment such as cyclones and bag collectors are used to separate solids from the gaseous stream. All of the steam generated is first condensed and is then recycled as water. According to the publication, the process is effective to separate and recover organic volatiles, semivolatiles, and other organic compounds from soils. Such a process as described is also subject to several disadvantages: i.e. the required cyclones and bag collectors are known to handle tacky materials such as wet sludges with difficulty; the boiler stack gases used in a plenum can easily educt volatile organics and semivolatile organics into this gas stream because one side of the plenum is under lower pressure. This can result in a violation of the applicable regulations and shutdowns. Up to four additional units may be required to precondition the contaminated soils and the soils may require conditioning to a 20 mesh or smaller size which is a significant size reduction operation in itself. High energy crushers and screening systems are necessary to accomplish these size requirements and additional air handling and treating equipment is also required to handle the dust and the organic emissions from the crushing and screening steps. Approximately 5 to 30% of the soils can remain in the process system because of the small particle size of the soils, resulting in line plugging, significant capacity reduction, and shutdown. Additionally, recycling water instead of steam makes the process significantly more expensive in energy costs, less efficient and more time consuming in view of the need to first condense all generated vapors and then to revaporize the liquids for use in the process.
There is presently no known method available for the cleanup of contaminated materials, without incineration, that is portable and compact, multi-stage, optionally continuous, at least as cost effective as the non-incineration conventional methods mentioned above, significantly more cost effective than methods which involve incineration, and free of the disadvantages associated with known thermal desorption methods.